Sb Mid-Infrared Quantum Dot Lasers
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Photonics 2015, 2, 414-425; doi:10.3390/photonics2020414 OPEN ACCESS photonics ISSN 2304-6732 www.mdpi.com/journal/photonics Article Gain and Threshold Current in Type II In(As)Sb Mid-Infrared Quantum Dot Lasers Qi Lu *, Qiandong Zhuang and Anthony Krier Physics Department, Lancaster University, Lancaster LA1 4YB, UK; E-Mails: [email protected] (Q.Z.); [email protected] (A.K.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +44-015-24-597-50; Fax: +44-015-24-593-651. Received: 27 February 2015 / Accepted: 11 April 2015 / Published: 15 April 2015 Abstract: In this work, we improved the performance of mid-infrared type II InSb/InAs quantum dot (QD) laser diodes by incorporating a lattice-matched p-InAsSbP cladding layer. The resulting devices exhibited emission around 3.1 µm and operated up to 120 K in pulsed mode, which is the highest working temperature for this type of QD laser. The modal gain was estimated to be 2.9 cm−1 per QD layer. A large blue shift (~150 nm) was observed in the spontaneous emission spectrum below threshold due to charging effects. Because of the QD size distribution, only a small fraction of QDs achieve threshold at the same injection level at 4 K. Carrier leakage from the waveguide into the cladding layers was found to be the main reason for the high threshold current at higher temperatures. Keywords: quantum dots; mid-infrared; semiconductor lasers 1. Introduction The mid-infrared wavelength range (2–5 µm) continues to attract significant research interest due to its importance for various potential applications, such as optical gas sensing, environmental pollution monitoring, chemical process control, non-invasive medical diagnosis, tunable IR spectroscopy, laser surgery and infrared countermeasures [1]. Efficient and cost-effective solid-state laser sources emitting in this spectral range are required in many applications. In recent years, mid-infrared quantum well (QW) lasers have made significant progresses in reducing the threshold current density, increasing maximum working temperature and output power [2,3]. Moreover, the quantum cascade lasers (QCLs) and inter-band cascade lasers (ICLs), which make use of the inter sub-band transition, have achieved Photonics 2015, 2 415 superior performance [4,5]. However, these types of devices suffer from lower wall plug efficiency and higher turn-on voltages than QW lasers. In addition, QCLs and ICLs are typically composed of hundreds of thin layers, making their growth more challenging. From theoretical studies, due to their discrete density of states, QDs have higher material gain, narrower gain spectrum, and are less temperature sensitive than QWs [6], making them the ideal candidate as the gain medium in semiconductor lasers. Compared with QCLs and ICLs, QD lasers also have the advantage of less complicated growth. Although the growth conditions for successful realization of QDs needs careful attention to detail to obtain the correct size distribution and area density, once these conditions are established the active region growth can be easily completed and does not require an extended growth period. Despite these potential merits, to date there are still very few reports on QD lasers in the mid-infrared range and their performance lags behind the aforementioned types of lasers. However, type II InSb/InAs QDs grown by molecular beam epitaxy (MBE), which have exhibited room temperature mid-infrared photo-luminescence (PL) [7] and electro-luminescence (EL) [8], could be a route towards QD light sources and lasers emitting beyond 3 µm. In this work, we present broad area InSb QD laser diodes grown on InAs substrates, which emitted around 3.1 µm. The structure incorporated a p-InAs0.61Sb0.13P0.26 cladding layer grown by liquid phase epitaxy (LPE). The laser diodes worked up to 120 K in pulsed mode, which is a significant improvement compared with previous work [9,10]. The gain of these type II QDs was estimated and the different contributions to the threshold current were studied. 2. Experimental Section The QD lasers were grown on (100) oriented p-InAs substrates. Firstly, a 2 µm thick InAs0.61Sb0.13P0.26 cladding layer was deposited by LPE from an indium-rich melt. The material was p-type doped with Zn to a concentration of ~2 × 1018 cm−3. The rest of the structure was then grown in a VG V80H MBE reactor. Ten layers of InSb QDs with 20 nm thick InAs spacers between each layer were deposited in the middle of a 1.2 µm thick undoped (1 × 1016 cm−3) InAs waveguide region. On top of that, an n-doped cladding layer was grown, which was composed of 0.3 µm thick n-InAs with a doping concentration of 1 × 1017 cm−3 and 1.7 µm thick n+-InAs with a doping concentration of 5 × 1018 cm−3 which had a lower refractive index to complete the waveguide. The structure and band alignment of the resulting lasers are sketched in Figure 1a. Current-voltage measurements from the fabricated lasers revealed good p-i-n diode rectifying characteristics. As shown in Figure 1b, the turn-on voltage decreased from 0.34 V at 4 K to 0.10 V at 300 K. The diode series resistance was ~2 Ω for all temperatures and the reverse leakage current was 4 mA (0.5 V reverse bias) at 300 K. Unlike the common Stranski-Krastanov growth of QDs, which mainly relies on the strain effect between the deposited material and the substrate [11], in this case the growth of the InSb QDs is based on the As-to-Sb anion exchange reaction, which is achieved by exposing the substrate to Sb2 flux for a very short time interval. The substrate temperature during dot formation can be adjusted in the interval 320–450 °C , resulting in InSb layers of thickness in the range 0.5–0.9 monolayer (ML). Ideally, larger QDs are required for use in lasers to maintain sufficient hole localization. For QDs grown using the exchange reaction, this is only possible at growth temperatures below 350 °C . However, it was found Photonics 2015, 2 416 that at growth temperatures in the range 320–350 °C the PL intensity decreased dramatically because of the poor material quality. Furthermore, strong Sb segregation can occur on the InAs surface forming rougher interfaces. Thus, to avoid this problem, in our laser structure the QDs were grown at higher substrate temperature (420 °C ). Then to increase the size of the QDs, InSb was deposited for 4 s following the Sb exchange reaction, resulting in QDs about 0.8 ML in thickness (i.e., a few nanometers in each dimension) but with a very high density (~1012 cm−2). More details of the QD exchange growth can be found in our earlier work [8]. In agreement with previous reports, no wetting layer was observed. The QDs grown using this method typically emitted in the 3–4 µm spectral range, and the emission wavelength was dependent upon the substrate temperature and Sb2 flux exposure time during the growth [8,9]. After growth, broad area laser stripes were fabricated from the sample using conventional photolithography and wet chemical etching. Ti/Au (20 nm/200 nm) metallization was thermally evaporated on both sides to form the ohmic contacts. The lasers were fabricated to be 75 µm in width and of selected varying cavity lengths. The diodes were mounted and wire-bonded on TO-46 headers for measurements and testing. Figure 1. (a) Illustration of the QD laser structure and the conduction and valence band alignments. (b) Current-voltage curves of the laser diode measured at 4 K and 300 K. 3. Results and Discussion 3.1. Gain from InSb QDs QD laser diodes with different cavity lengths were tested in pulsed mode (100 ns pulse width, 2 kHz −2 repetition rate) from 4 K to 120 K. The threshold current density (Jth) increased from 1.59 kAcm to 4.68 kAcm−2 as the cavity length was reduced from 1.17 mm to 0.33 mm. Using the same method as for QW lasers [12], the gain of our InSb QD lasers was estimated from the relation between Jth and different laser cavity lengths L, using: 1 1 훼 + 푙푛 ( ) 푛푤퐽푡푟 퐿 푅 퐽푡ℎ = 푒푥푝 ( ) (1) 휂 푛푤Γ푤퐺0 Photonics 2015, 2 417 where nw is the number of QD layers, Jtr is the transparency current density per layer, ηi is the internal quantum efficiency, αi is the waveguide loss, R is the reflection coefficient of the end facet, ΓwG0 is the modal gain in which Γw is the optical confinement factor per layer and G0 is the material gain coefficient. By making a linear fit between ln (Jth) and 1/L, the modal gain can be extracted. Figure 2. Threshold current density (Jth) vs. reciprocal cavity length (1/L) of the laser diodes at 4 K. The red line shows the linear fit between ln(Jth) and 1/L. From Figure 2, the modal gain of the QD laser was estimated to be ~29 cm−1, (i.e., 2.9 cm−1 per QD layer on average). This value is lower than typical (InAs) type I QDs emitting in the near-infrared range which is in the order of 10 cm−1 [13,14], but is close to that of type II QWs emitting at a similar wavelength [12]. The relatively lower modal gain from type II structures is closely related to the large spatial separation between electrons and holes compared with type I structures, which results in a much smaller electron-hole wave function overlap.